Glass sheets which are press formed individually for making laminated backlites or windshields are cooled in such a manner as to control stresses. This cooling is accomplished either by natural convection or by low level forced air cooling. Such lites develop surface stresses which are comparable to those of actively annealed glass sheets from conventional, gravity sagged windshield processes.
However, the edge compression stress on press formed lites cooled by natural convection or low level forced cooling is generally higher than that for conventional, actively annealed lites. Edge compression is typically 300 to 400 Kg/cm.sup.2 and 150 to 200 Kg/cm.sup.2 for press formed lites cooled by natural convection or low level forced cooling and conventional actively annealed lites, respectively. This higher edge compression on the pressed formed lites represents an improvement in that said lites are less susceptible to edge breakage from handling.
Inboard from the edge compression, however, is a band of tension. The integrated inboard tension must equal the integrated edge compression. Therefore, the inboard tension is necessarily higher for individually pressed formed and cooled lites than for conventional gravity sagged lites. The measurable net inner band tension (NIBT) on press formed and cooled lites is typically 50 to 120 Kg/cm.sup.2, where on conventional gravity sagged lites it is typically 25 to 50 Kg/cm.sup.2.
Experience has shown that levels of net inner band tension in excess of about 60 Kg/cm.sup.2 can result in performance problems (breakage) for windshield lites. Specifically, laminated windshields with inner band tension in excess of 60 Kg/cm.sup.2 do not pass the so-called "scratch test" for windshields.
In the scratch test, the laminate (or individual lite) is abraded using 80 grit aluminum oxide sandpaper. The area typically abraded is a band about 6 inches wide immediately inside the perimeter of the glass. If cracking occurs within 24 hours of the abrasion, the sample is said to fail the scratch test. Experience has shown that samples with NIBT in excess of 60 Kg/cm.sup.2 usually fail the test while those with less NIBT do not. The test is considered a good measure of the propensity of an installed windshield to crack from incidental abrasion or from stone impact.
Attempts have been made to reduce the magnitude of NIBT by reducing the rate of cooling of the glass sheet, especially inboard of where the glass sheet contacts a support ring.
Experience has also shown that edge compression of less than about of 150 to 200 Kg/cm.sup.2 can result in breakage of laminated glass during installation of said glass into its opening, typically in an automobile. Higher edge compression will reduce the propensity of breakage during installation.
U.S. Pat. No. 5,069,703 discloses a covering for the tempering of glass sheets wherein a metallic fabric is used to cover a frame intended to support glass sheets being annealed. The metallic fabric is of low thermal conductivity and reduces the heat sink characteristics of a typical frame.
U.S. Pat. No. 4,687,501 discloses lightweight bending iron heat shields for glass bending molds that shade glass sheets from a heat source. The heat shields are utilized to change the heating rate of different areas of the glass sheets to effect the final shape of the glasses.
Typical glass sheets cooled by natural convection or low level forced cooling on conventional annealing rings have excess net inner band tension.